Ion implantation is one of the key technologies in the fabrication of semiconductor integrated circuit devices. With current ion implanters, a very accurate and extremely pure dose of a desired atomic species can be introduced into a target material. Photoresist is typically used as a masking structure for controlling where implants are to be located. More particularly, a layer of photoresist is deposited, exposed through a photo mask and developed to produce a photoresist structure having openings at locations where implants are to be located.
One type of ion implant, commonly referred to as an “angled implant” involves implantation of species at an angle that is not orthogonal to the top surface of the semiconductor substrate. Angled implants are often used in semiconductor fabrication processes to minimize the channeling effect in order to place dopants at appropriate positions within the semiconductor substrate. However, the photoresist masking structure produces an implant “shadow” in angled implants. That is, the photoresist structure obstructs implantation within an area commonly referred to as a shadow area. Shadow area limits reduction in size of semiconductor devices and limits flexibility of the device design process.
The location and size of the shadow area depends on the direction of the implant, the implant angle, the height of the photoresist structure, and the photoresist profile. Shadow area could be reduced by depositing a layer of photoresist having the minimum thickness required for each particular ion implantation process. However, because of resist management practices that limit the use of resists having different viscosities and equipment capacity limitations that do not allow for precise control of the thickness of the photoresist layer, the layer of photoresist is often much thicker than the basic blockage requirement for a particular implantation process.
Accordingly, there is a need for a method for reducing shadow area. Also, there is a need for a method for forming a photoresist structure that reduces shadow area. In addition, there is a need for a method for forming an implant region in which shadow area is reduced. The present invention meets the above needs.